Year	Number of
1990	3.12
1991	3.14
1992	3.15
1993	3.14
1994	3.15
1995	3.17
1996	3.19
1997	3.21
1998	3.2
1999	3.15
2000	3.29
2001	3.37
2002	3.41
2003	3.5
2004	3.6
2005	3.73
2006	3.81
2007	3.84
2008	3.83
2009	3.83
2010	3.86
2011	3.9
2012	3.92
2013	3.96
2014	4
2015	4.05
2016	4.08